1. Field of the Invention
The present invention relates to a coupling circuit.
2. Description of the Prior Art
Transducers whose sensitivity varies with temperature are widely used in industry. In general, it is necessary to provide some form of compensation for the temperature variation of the sensitivity. For instance, piezo-resistive bridge transducers when used in vehicles are subjected to a very wide range of temperatures, and compensation for the variation of sensitivity with temperature is essential in order for such transducers to provide a useful indication of a measured parameter.
GB2193577 discloses a semi-conductor pressure sensor in the form of a strain gauge bridge arrangement whose sensitivity falls as the temperature of the strain gauge elements increases. In order to compensate for this, the strain gauge bridge is supplied with a "constant" current which is arranged to increase with temperature so as to compensate at least partially for the fall in sensitivity with temperature. The current is supplied from the output of an operational amplifier whose non-inverting input is connected to a voltage reference in the form of a potential divider, one arm of which includes a thermistor.
A problem with this arrangement is that the output of the operational amplifier is required to supply a substantial current through the relatively low effective resitance of the bridge and, in order to do this, the output of the operational amplifier must be capable of approaching the power supply rail closely. However, when the output of an operational amplifier approaches the power supply line, the output current is limited and may well be less than that required to operate the bridge properly. Conversely, the relatively large output current which the bridge attempts to draw from the output of the operational amplifier causes a substantial voltage drop between the power supply line and the operational amplifer output. The result of this is to limit the effective range of the temperature compensation, and this can limit the effective use of the transducer, for instance where the power supply voltage is restricted. This is the case in automotive applications where the maximum power supply voltage is limited by the voltage of the vehicle battery and, in practice, may well have to be substantially less than this to allow for voltage regulation to ensure that automotive electronic circuits function correctly.
A bridge transducer, for instance of the piezo-resistive type, is generally connected to a differential amplifier whose output provides a signal which is a measure of the parameter sensed by the transducer. However, the output signal from such an amplifier generally has a direct component, which may vary with temperature and in some cases it is desirable to remove this before making use of the transducer signal. Where direct coupling is undesirable, AC coupling is normally employed to remove the direct component. However, because the desired varying component of the transducer signal can have components of very low frequency and such components must not be unduly attenuated by the coupling arrangement, resistance-capacitance coupling networks having very large resistance-capacitance products have to be employed. This results in the disadvantage of a relatively long settling time when power :s applied to such circuit arrangements In particular, the capacitor has to be charged through the resistor of the coupling arrangement and the vary long RC time constant necessary to avoid undesirable low frequency attenuation can render the output signal from the transducer unusable for a substantial period of time until the capacitor has been sufficiently charged.